Question: The scale on a map is 5cm : 2km. If the distance between two cities is 6km, how far apart in cm are the two cities on the map?
Answer: The scale means that every 5cm on the map corresponds to an actual distance of 2km. An actual distance of 6km is the same as 3 $\cdot$ 2km. The distance between the two cities on the map is 3 $\cdot$ 5cm, or 15cm.